Now collecting 'perks' - special, replay-only bennies, that stack over time - this sounds more interesting to me than collecting skills from the various classes. That's an idea I could really get behind.

It gives the classes a firmer identity and flavor (since the only way you could get a wind warrior, for instance, would be by being a psion), but still give you benefits and incentive to play, and replay.

As a person who's never played fallout, "perk" felt like a perfectly appropriate word. But if you don't like it, there are alternatives: "fringe benefit" "lagniappe", "honorarium" to name three pulled randomly from thesaurus.com.